Solar heat collector tube and production method thereof

ABSTRACT

A solar heat collector tube in which at least an infrared reflective layer, a sunlight-heat conversion layer and an anti-reflection layer are provided on the outer surface of a tube, through the interior of which a heat medium can flow, wherein the infrared reflective layer is an Ag layer in which silicon, silicon nitride or a mixture thereof is dispersed, and a method for producing the solar heat collector tube wherein the infrared reflective layer that is an Ag layer, in which silicon, silicon nitride or a mixture thereof is dispersed, is formed by sputtering in the presence of a gas including nitrogen gas, with Ag and silicon being used as targets.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a National Stage of International Application No.PCT/JP2016/084339 filed Nov. 18, 2016, claiming priority based onJapanese Patent Application No. 2016-015519 filed Jan. 29, 2016.

TECHNICAL FIELD

The present invention relates to a solar heat collector tube and aproduction method thereof.

BACKGROUND ART

Solar heat power generation apparatuses that convert sunlight to heatand generate power utilizing that heat are known. In these apparatuses,sunlight is condensed by condensing means, a heat medium inside a solarheat collector tube is heated by the condensed sunlight, and thermalenergy of the heat medium having been thus heated is utilized togenerate power. Such apparatuses utilize therefore a solar heatcollector tube in which various layers for efficiently convertingsunlight to heat are formed on the outer surface of a tube, through theinterior of which the heat medium can flow. On the outer surface of atube, through the interior of which a heat medium can flow, there are,for instance, formed an infrared reflective layer that reflects thermalradiation from the medium and the tube, a sunlight-heat conversion layerthat converts sunlight to heat, and an anti-reflection layer thatprevents reflection of sunlight. Among these various layers, using an Aglayer on the infrared reflective layer, is well known (see for instancePatent Literature 1).

CITATION LIST Patent Literature

[Patent Literature 1] Japanese Patent Application Publication No.2010-271033

SUMMARY OF INVENTION Technical Problem

When the temperature of the heat medium flowing through the interior ofthe solar heat collector tube increases, the temperature of the outersurface of the tube having the heat medium flowing therethrough becomesaround 650° C. to 700° C., and also the infrared reflective layer formedon the outer surface of the tube becomes exposed to a high temperatures.Herein Ag layers conventionally used as the infrared reflective layer donot have sufficient heat resistance, and as a result Ag suffersaggregation and sublimation, in about 1 hour, when exposed to hightemperature, and the effect of reflecting thermal radiation from theheat medium and the tube is weakened. Thus the efficiency of convertingsunlight to heat decreases, since the Ag layer in such a state cannotsufficiently function as an infrared reflective layer.

In order to solve the above problem, it is an object of the presentinvention to provide a solar heat collector tube and a production methodthereof in which efficiency of conversion of sunlight to heat does notdrop readily, through the use, in an infrared reflective layer, of an Aglayer having excellent heat resistance and in which aggregation andsublimation of Ag can be suppressed, even upon exposure to hightemperatures.

Solution to Problem

As a result of diligent research aimed at solving the above problem, theinventors found that aggregation and sublimation of Ag can be suppressedby dispersing silicon, silicon nitride or a mixture thereof in the Aglayer, and perfected the present invention on the basis of that finding.

Specifically, the present invention is a solar heat collector tube inwhich at least an infrared reflective layer, a sunlight-heat conversionlayer and an anti-reflection layer are provided on the outer surface ofa tube, through the interior of which a heat medium can flow, whereinthe infrared reflective layer is an Ag layer in which silicon, siliconnitride or a mixture thereof is dispersed.

Also, the present invention is a method for producing a solar heatcollector tube in which at least an infrared reflective layer, asunlight-heat conversion layer and an anti-reflection layer are providedon the outer surface of a tube, through the interior of which a heatmedium can flow, the method including: forming the infrared reflectivelayer that is an Ag layer in which silicon, silicon nitride or a mixturethereof is dispersed, by sputtering in the presence of a gas includingnitrogen gas, with Ag and silicon being used as targets.

Advantageous Effects of Invention

The present invention succeeds in providing a solar heat collector tubeand a production method thereof in which efficiency of conversion ofsunlight to heat does not drop readily, through the use, in an infraredreflective layer, of an Ag layer having excellent heat resistance and inwhich aggregation and sublimation of Ag can be suppressed, even uponexposure to high temperatures.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partial cross-sectional diagram of a solar heat collectortube of Embodiment 1.

FIG. 2 is a scanning electronic microscope (SEM) micrograph of an Aglayer made of Ag alone and formed on a quartz substrate, after havingbeen heated for 1 hour at 700° C.

FIG. 3 illustrates results of light transmittance of an Ag layer made ofAg alone and formed on a quartz substrate, before and after 1 hour ofheating at 700° C.

FIG. 4 illustrates a scanning electronic microscope (SEM) micrograph ofan Ag layer formed on a quartz substrate and having silicon, siliconnitride or a mixture thereof dispersed therein, after heating at 700° C.for 1 hour.

FIG. 5 illustrates results of light transmittance of an Ag layer formedon a quartz substrate and having silicon, silicon nitride or a mixturethereof dispersed therein, before and after heating the Ag layer at 700°C. for 1 hour.

FIG. 6 is a partial cross-sectional diagram of a solar heat collectortube of Embodiment 2.

FIG. 7 is a partial cross-sectional diagram of a solar heat collectortube of Embodiment 3.

FIG. 8 is a partial cross-sectional diagram of a solar heat collectortube of Embodiment 4.

FIG. 9 is a stack resulting from sequential layering, on a quartzsubstrate, of a metal protective layer (20 nm W layer), an Ag layer (230nm) having dispersed therein 1.25 at % of silicon, silicon nitride or amixture thereof, a metal protective layer (5 nm W layer) and an oxygenbarrier layer (50 nm Si₃N₄ layer).

FIG. 10 illustrates results of light transmittance of the stack of FIG.9 before and after heating of the stack at 700° C. for 1 hour, 11 hoursand 51 hours.

FIG. 11 is a partial cross-sectional diagram of a solar heat collectortube of Embodiment 5.

FIG. 12 is a partial cross-sectional diagram of a solar heat collectortube of Embodiment 6.

FIG. 13 is a partial cross-sectional diagram of a solar heat collectortube of Embodiment 7.

FIG. 14 is a stack resulting from sequential layering, on a quartzsubstrate, of a reaction preventing layer (20 nm TaSi₂ layer), a metalprotective layer (20 nm Ta layer), an Ag layer (230 nm) having dispersedtherein 1.25 at % of silicon, silicon nitride or a mixture thereof, ametal protective layer (10 nm Ta layer), a reaction preventing layer (10nm TaSi₂ layer) and an oxygen barrier layer (50 nm Si₃N₄ layer).

FIG. 15 illustrates results of light transmittance of the stack of FIG.14 before and after heating of the stack at 700° C. for 1 hour, 11 hoursand 51 hours.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the solar heat collector tube and productionmethod thereof of the present invention will be explained next withreference to accompanying drawings.

Embodiment b 1

FIG. 1 is a partial cross-sectional diagram of a solar heat collectortube of the present embodiment.

In FIG. 1 a solar heat collector tube 1 of the present embodiment has atube 2, through the interior of which a heat medium can flow, aninfrared reflective layer 3 formed on the outer surface of the tube 2, asunlight-heat conversion layer 4 formed on the infrared reflective layer3, and an anti-reflection layer 5 formed on the sunlight-heat conversionlayer 4.

The tube 2 through the interior of which a heat medium can flow is notparticularly limited, and tubes known in the relevant technical fieldcan be used herein. Ordinarily, a metal having heat resistance, asrepresented by an iron-based material (for instance, stainless steel,heat-resistant steel, alloy steel or carbon steel) or an aluminum-basedmaterial, can be used as the material of the tube 2. Taking the usageenvironment (for instance, the heating temperature of the tube 2) intoaccount, the tube 2 is preferably made of stainless steel orheat-resistant steel.

The heat medium that flows through the interior of the tube 2 is notparticularly limited, and heat media known in the relevant technicalfield can be used herein. Examples of the heat medium include forinstance water, oil, molten salts (for instance molten sodium) and thelike.

The infrared reflective layer 3 formed on the outer surface of the tube2 has the function of reflecting thermal radiation (heat radiation) fromthe heat medium and the tube 2. The heat medium used in the solar heatcollector tube 1 and the materials in the tube 2 and so forth may insome instances become heated to a high temperature of around 650° C. to700° C., in which case most of the emitted electromagnetic waves areinfrared rays. Accordingly, the main function of the infrared reflectivelayer 3 is to reflect these infrared rays. Specifically, the infraredreflective layer 3 suppresses emission, towards the exterior of the tube2, of thermal energy given off by the heat medium and the tube 2.

Conventionally an Ag layer 7 has been used as the infrared reflectivelayer 3. However, an Ag layer 7 made up of only Ag exhibits aggregationor sublimation of Ag, after about 1 hour, when exposed at a hightemperature of around 650° C. to 700° C.

FIG. 2 illustrates a scanning electronic microscope (SEM) micrograph ofan Ag layer 7 made of Ag alone and formed on a quartz substrate, afterhaving been heated for 1 hour at 700° C. FIG. 3 illustrates results oflight transmittance of the Ag layer 7 before and after heating.

As depicted in FIG. 2, Ag in the Ag layer 7 sublimates and aggregatesdue to heating, and the quartz substrate that underlies the lower Aglayer 7 becomes exposed. as illustrated in FIG. 3, the Ag layer 7 beforeheating has substantially zero light transmittance at a wavelengthregion from about 200 nm to 2500 nm (no light is transmitted within thiswavelength region), whereas the Ag layer 7 after heating exhibits alight transmittance of about 40% in the wavelength region from about 200nm to 2500 nm (light is transmitted in this wavelength region). Thefunction as the infrared reflective layer 3 (function of reflectingthermal radiation from the heat medium and the tube) is thus notsufficiently brought out by the Ag layer 7 having suffered aggregationand sublimation of Ag, and accordingly the effect in conversion ofsunlight to heat is low.

In the solar heat collector tube 1 of the present embodiment, therefore,an Ag layer 7 having dispersed therein silicon, silicon nitride or amixture thereof 6 is used as the infrared reflective layer 3. Herein thesilicon, silicon nitride or a mixture thereof 6 has the function ofsuppressing aggregation and sublimation of Ag in the Ag layer 7, as aresult of which there is enhanced the heat resistance of the Ag layer 7.

FIG. 4 illustrates a scanning electronic microscope (SEM) micrograph ofan Ag layer 7 formed on a quartz substrate and having dispersed therein1.25 at % of silicon, silicon nitride or a mixture thereof 6, afterheating at 700° C. for 1 hour. FIG. 5 illustrates results of lighttransmittance of the Ag layer 7 before and after heating at 700° C. for1 hour.

As FIG. 4 illustrates, the quartz substrate underlying the Ag layer 7does not become exposed, and there is virtually no aggregation orsublimation of Ag, even after heating at 700° C. for 1 hour. Moreover,the light transmittance of the Ag layer 7 exhibits virtually no changebefore or after heating for 1 hour, as illustrated in FIG. 5. Therefore,the Ag layer 7 having dispersed therein the silicon, silicon nitride ora mixture thereof 6 allows suppressing aggregation and sublimation ofAg, even when exposed to a high temperature of about 700° C.Accordingly, the function of the Ag layer 7 as an infrared reflectivelayer 3 (function of reflecting thermal radiation from the heat mediumand the tube) is not impaired, and the efficiency of conversion ofsunlight to heat does not drop.

The amount of the silicon, silicon nitride or a mixture thereof 6dispersed in the Ag layer 7 is not particularly limited, but ispreferably lower than 10 at %, and is more preferably 0.1 at % to 5 at%, yet more preferably 0.3 at % to 3 at %, and particularly preferably0.5 at % to 2 at %.

The thickness of the Ag layer 7 having dispersed therein the silicon,silicon nitride or a mixture thereof 6 is not particularly limited, butis preferably 10 nm to 500 nm, more preferably 30 nm to 400 nm, and yetmore preferably 50 nm to 300 nm.

The Ag layer 7 having dispersed therein the silicon, silicon nitride ora mixture thereof 6 can be formed by sputtering in the presence of a gasincluding argon gas or nitrogen gas, with Ag and silicon being used astargets. The sputtering conditions are not particularly limited, and maybe adjusted as appropriate depending on the apparatus that is utilized.As the target, individual targets of Ag and silicon, or one target inthe form of a mixture of Ag and silicon can be used.

The Ag layer 7 may have further dispersed therein at least one metalselected from the group consisting of Mo, W, Ta, Nb and Al, in additionto the silicon, silicon nitride or a mixture thereof 6. Similarly to thesilicon, silicon nitride or a mixture thereof 6, the at least one metalselected from the group consisting of Mo, W, Ta, Nb and Al elicits theeffect of suppressing aggregation and sublimation of Ag in the Ag layer7, and as a result also enhancing the heat resistance of the Ag layer 7.

In a case where the Ag layer 7 has dispersed therein the silicon,silicon nitride or a mixture thereof 6 and at least one metal selectedfrom the group consisting of Mo, W, Ta, Nb and Al, the amount of thesilicon, silicon nitride or a mixture thereof 6 in the Ag layer 7 is notparticularly limited, but is preferably lower than 10 at %, and is morepreferably 2 at % to 4 at %, while the amount of the at least one metalselected from the group consisting of Mo, W, Ta, Nb and Al in the Aglayer 7 is not particularly limited, but is preferably lower than 10 at%, and is more preferably 7 at % to 9 at %.

The thickness of the Ag layer 7 having dispersed therein the silicon,silicon nitride or a mixture thereof 6 and at least one metal selectedfrom the group consisting of Mo, W, Ta, Nb and Al is not particularlylimited, but is preferably 10 nm to 500 nm, and more preferably 50 nm to300 nm.

The Ag layer 7 having dispersed therein the silicon, silicon nitride ora mixture thereof 6 and the at least one metal selected from the groupconsisting of Mo, W, Ta, Nb and Al can be formed by sputtering in thepresence of a gas including argon gas or nitrogen gas, using Ag, siliconand at least one metal selected from the group consisting of Mo, W, Ta,Nb and Al as targets. The sputtering conditions are not particularlylimited, and may be adjusted as appropriate depending on the apparatusthat is utilized. The target may be individual targets of Ag, ofsilicon, and of at least one metal selected from the group consisting ofMo, W, Ta, Nb and Al, or one target in the form of a mixture of Ag,silicon and at least one metal selected from the group consisting of Mo,W, Ta, Nb and Al.

The sunlight-heat conversion layer 4 formed on the infrared reflectivelayer 3 has the function of efficiently absorbing sunlight whilesuppressing heat dissipation by thermal radiation. The sunlight-heatconversion layer 4 is also referred to as a light-selective absorptionlayer.

A sunlight-heat conversion layer known in the relevant technical fieldcan be used, without particular limitations, as the sunlight-heatconversion layer 4. Examples of the sunlight-heat conversion layer 4include, for instance, a black chromium plating layer, a black nickelplating layer, an electroless nickel blackened layer, a triirontetraoxide (Oxide black) layer, a cermet layer (layer made up of acomposite material of a ceramic and a metal), an iron silicide layer, amanganese silicide layer, a chromium silicide layer, or a layer made upof a composite material of manganese silicide or chromium silicide and atransparent dielectric (for instance, SiO₂, Al₂O₃, AlN or the like). Theforegoing layers may be single layers or a plurality of layers of two ormore types.

The thickness of the sunlight-heat conversion layer 4 is notparticularly limited, but is preferably 1 nm to 10 and more preferably 5nm to 100 nm.

The method for forming the sunlight-heat conversion layer 4 is notparticularly limited, and a method known in the relevant technical fieldcan be resorted to. For instance, the sunlight-heat conversion layer 4can be formed by chemical vapor deposition, physical vapor deposition(sputtering, vacuum deposition, ion plating or the like), or by plating.

The anti-reflection layer 5 formed on the sunlight-heat conversion layer4 has the function of reflecting sunlight.

An anti-reflection layer known in the relevant technical field can beused, without particular limitations, as the anti-reflection layer 5.Examples of the anti-reflection layer 5 include, for instance,transparent dielectric layers such as such as SiO₂ layers, Al₂O₃ layers,AlN layers, Cr₂O₃ layers and the like.

The thickness of the anti-reflection layer 5 is not particularlylimited, but is preferably 10 nm to 500 nm.

The method for forming the anti-reflection layer 5 is not particularlylimited, and a method known in the relevant technical field can beresorted to. For instance, the anti-reflection layer 5 can be formed bychemical vapor deposition or physical vapor deposition (sputtering,vacuum deposition or ion plating).

Through dispersion of the silicon, silicon nitride or a mixture thereof6 in the Ag layer 7, the solar heat collector tube 1 of the presentembodiment having such features is thereby provided with an infraredreflective layer 3 that suppresses aggregation and sublimation of Ag,and accordingly drops in the efficiency of conversion of sunlight toheat become less likely.

Embodiment 2

FIG. 6 is a partial cross-sectional diagram of a solar heat collectortube of the present embodiment.

In FIG. 6, a solar heat collector tube 10 of the present embodimentdiffers from the solar heat collector tube 1 of Embodiment 1 in that ametal protective layer 11 is provided between the infrared reflectivelayer 3 and the sunlight-heat conversion layer 4. Other features areidentical to those of the solar heat collector tube 1 of Embodiment 1,and accordingly will not be explained.

The metal protective layer 11 has the function of making Ag contained inthe infrared reflective layer 3 less likely to sublimate. Accordingly,sublimation of Ag contained in the infrared reflective layer 3 can befurther suppressed, and impairment of the function of the infraredreflective layer 3 made less likely, by formation of the metalprotective layer 11 between the infrared reflective layer 3 and thesunlight-heat conversion layer 4.

The metal protective layer used in the metal protective layer 11 is notparticularly limited so long as the layer has the function of making Agless likely to sublimate, and generally the layer is a metal protectivelayer formed out of a material of a higher melting point than that of Ag(melting point 9618° C.)

Examples of materials having a melting point higher than that of Aginclude, for instance, Nb (melting point 2469° C.), Mo (melting point2623° C.), W (melting point 3422° C.), Cu (melting point 1085° C.), Ni(melting point 1455° C.), Fe (melting point 1538° C.), Cr (melting point1907° C.), Ta (melting point 3020° C.) and the like.

In a case where at least one metal selected from the group consisting ofMo, W, Ta, Nb and Al is dispersed in the Ag layer 7, the metalprotective layer 11 is preferably formed out of a material that containsthe metal dispersed in the Ag layer 7 (i.e. at least one metal selectedfrom the group consisting of Mo, W, Ta, Nb and Al). A compound ofsilicon or nitrogen and at least one metal selected from the groupconsisting of Mo, W, Ta, Nb and Al can be used as such a material.Examples of such compounds include, for instance, TaSi₂ (melting point2200° C.), MoSi₂ (melting point 2020° C.) Mo₅Si₃ (melting point 2180°C.), WSi₂ (melting point 2160° C.), TaN (melting point 3083° C.), NbSi₂(melting point 1930° C.), NbN (melting point 2300° C.) and the like.

Preferably, the material that forms the metal protective layer 11 hashigh reflectance towards light in the infrared region. For instance, Nbhas a reflectance of 96.1%, Mo of 97.1%, W of 95.2%, Cu of 97.4%, Ni of86.4%, Fe of 81.8%, Cr of 81.3%, and Ta of 97.3%, towards infrared lightat a wavelength of 2500 nm; preferred herein are thus Ta, Nb, Mo, W andCu having a reflectance towards light in the infrared region in excessof 90%.

The thickness of the metal protective layer 11 may be set as appropriatefor instance depending on the type of material that is used and is notparticularly limited, but is preferably smaller than the thickness ofthe infrared reflective layer 3, from the viewpoint of suppressingthermal radiation.

An appropriate thickness of the metal protective layer 11 formed on theinfrared reflective layer 3 may be worked out by calculating emissivityon the basis of the results of a multilayer film approximation using theoptical constants of the materials that are utilized in the infraredreflective layer 3 and the metal protective layer 11. In a case, forinstance, where the metal protective layer 11 is formed using Mo on theinfrared reflective layer 3 having a thickness of 100 nm and comprising1.25 at % of silicon, silicon nitride or a mixture thereof 6, emissivityat 650° C. can be made lower than that of a Cu layer by prescribing thethickness of the metal protective layer 11 (Mo layer) to lie in therange of 0.1 nm to 39.5 nm. In a case where the metal protective layer11 is formed using W, emissivity at 650° C. can be made lower than thatof a Cu layer by prescribing the thickness of the metal protective layer11 (W layer) to lie in the range of 0.1 nm to 14.1 nm. In a case wherethe metal protective layer 11 is formed using Nb, emissivity at 650° C.can be made lower than that of a Cu layer by prescribing the thicknessof the metal protective layer 11 (Nb layer) to lie in the range of 0.1nm to 5.6 nm.

The method for forming the metal protective layer 11 is not particularlylimited, and a method known in the relevant technical field can beresorted to. For instance, the metal protective layer 11 can be formedby chemical vapor deposition or physical vapor deposition (sputtering,vacuum deposition or ion plating).

A solar heat collector tube 10 of the present embodiment having suchfeatures allows yet further suppression of sublimation of Ag containedin the infrared reflective layer 3, and accordingly impairment of thefunction of the infrared reflective layer 3 becomes yet less likely.Therefore, drops in efficiency of conversion of sunlight to heat are yetless likely in the solar heat collector tube 10.

Embodiment 3

FIG. 7 is a partial cross-sectional diagram of a solar heat collectortube of the present embodiment.

In FIG. 7 a solar heat collector tube 20 of the present embodimentdiffers from the solar heat collector tube 10 of Embodiment 2 in that ametal protective layer 11 is further provided between the tube 2 and theinfrared reflective layer 3. Other features are identical to those ofthe solar heat collector tube 10 of Embodiment 2, and accordingly willnot be explained. The features of the present embodiment can apply alsoto the solar heat collector tube 1 of Embodiment 1.

The metal protective layer 11 provided between the tube 2 and theinfrared reflective layer 3 is provided as the underlying base of theinfrared reflective layer 3, and has the function of facilitatinguniform formation of the infrared reflective layer 3. Accordingly, theinfrared reflective layer 3 can be formed uniformly, and the function ofthe infrared reflective layer 3 can be obtained stably, by formation ofthe metal protective layer 11 between the tube 2 and the infraredreflective layer 3.

The metal protective layer 11 that is provided between the tube 2 andthe infrared reflective layer 3 is not particularly limited, and therecan be used the same metal protective layer 11 that is provided on theinfrared reflective layer 3.

The thickness of the metal protective layer 11 provided between the tube2 and the infrared reflective layer 3 is not particularly limited, solong as the function of the metal protective layer 11 as an underlyingbase can be brought out, but is generally 1 nm to 100 nm, preferably 3nm to 50 nm, and more preferably 5 nm to 30 nm.

In addition to the effect of the solar heat collector tube 1 ofEmbodiment 1 or the solar heat collector tube 10 of Embodiment 2, thesolar heat collector tube 20 of the present embodiment having suchfeatures allows the function of the infrared reflective layer 3 to beachieved.

Embodiment 4

FIG. 8 is a partial cross-sectional diagram of a solar heat collectortube of the present embodiment.

In FIG. 8, a solar heat collector tube 30 of the present embodimentdiffers from the solar heat collector tube 20 of Embodiment 3 in that anoxygen barrier layer 12 is provided between the metal protective layer11 and the sunlight-heat conversion layer 4. Other features areidentical to those of the solar heat collector tube 20 of Embodiment 3,and accordingly will not be explained. The features of the presentembodiment can apply also to the solar heat collector tube 10 ofEmbodiment 2.

The oxygen barrier layer 12 is provided for the purpose of preventingpassage of oxygen, which gives rise to oxidation of the metal protectivelayer 11. Accordingly, oxidation of the metal protective layer 11 can beprevented by formation of the oxygen barrier layer 12 between the metalprotective layer 11 and the sunlight-heat conversion layer 4, andaccordingly impairment of the function of the metal protective layer 11becomes less likely.

The oxygen barrier layer 12 is not particularly limited and any oxygenbarrier layer can be employed so long as the passage of oxygen thereofis not easy, and, for instance, a dielectric layer can be used as theoxygen barrier layer 12. Examples of dielectric layers include, forinstance, SiO₂ layers, Al₂O₃ layers, AlN layers, Cr₂O₃ layers and Si₃N₄layers.

The thickness of the oxygen barrier layer 12 is not particularlylimited, so long as passage of oxygen is precluded, but is generally 1nm to 100 nm, preferably 3 nm to 50 nm, and more preferably 5 nm to 30nm.

The method for forming the oxygen barrier layer 12 is not particularlylimited, and a method known in the relevant technical field can beresorted to. For instance, the oxygen barrier layer 12 can be formed bychemical vapor deposition or physical vapor deposition (sputtering,vacuum deposition or ion plating).

Here, the stack in FIG. 9 is produced through sequential layering, on aquartz substrate, of the metal protective layer 11 (20 nm W layer), theAg layer 7 (230 nm) having dispersed therein 1.25 at % of silicon,silicon nitride or a mixture thereof 6, the metal protective layer 11 (5nm W layer) and the oxygen barrier layer 12 (50 nm Si₃N₄ layer). FIG. 10illustrates the results of light transmittance of the stack before andafter heating of the stack at 700° C. for 1 hour, 11 hours and 51 hours.As FIG. 10 reveals, the light transmittance of the stack exhibitsvirtually no change before or after heating. By adopting such amultilayer structure, therefore, the functions of the various layers arenot impaired and the efficiency of conversion of sunlight to heat doesnot drop.

In addition to the effect of the solar heat collector tube 10 ofEmbodiment 2 and the effect of the solar heat collector tube 20 ofEmbodiment 3, the solar heat collector tube 30 of the present embodimenthaving such features allows prevention of impairment in the function ofthe metal protective layer 11 caused by oxidation thereof.

Embodiment 5

FIG. 11 is a partial cross-sectional diagram of solar heat collectortube of the present embodiment.

In FIG. 11, a solar heat collector tube 40 of the present embodimentdiffers from the solar heat collector tube 30 of Embodiment 4 in thatherein a diffusion preventing layer 13 is provided between the tube 2and the infrared reflective layer 3. Other features are identical tothose of the solar heat collector tube 30 of Embodiment 4, andaccordingly will not be explained. The features of the presentembodiment can also apply to the solar heat collector tube 1 ofEmbodiment 1, the solar heat collector tube 10 of Embodiment 2 and thesolar heat collector tube 20 of Embodiment 3.

The diffusion preventing layer 13 is provided for the purpose ofpreventing components of the tube 2 (for instance, Cr) from diffusinginto the layer (metal protective layer 11 in FIG. 11) overlying the tube2. Impairment of the function of the layer (metal protective layer 11 inFIG. 11) overlying the tube 2 becomes less likely by formation of thediffusion preventing layer 13 on the outer surface of the tube 2.

A diffusion preventing layer known in the relevant technical field canbe used, without particular limitations, as the diffusion preventinglayer 13. Examples of materials used in the diffusion preventing layer13 include, for instance, oxides such as SiO₂ and Al₂O₃, and nitridessuch as Si₃N₄ and AlN.

The thickness of the diffusion preventing layer 13 is not particularlylimited, so long as components in the tube 2 can be prevented fromdiffusing into the layer above, but is generally of 1 nm to 100 nm,preferably 3 nm to 50 nm, and more preferably 5 nm to 30 nm.

The method for forming the diffusion preventing layer 13 is notparticularly limited, and a method known in the relevant technical fieldcan be resorted to. For instance, the diffusion preventing layer 13 canbe formed by chemical vapor deposition or physical vapor deposition(sputtering, vacuum deposition or ion plating).

In addition to the effects of the solar heat collector tube 1 ofEmbodiment 1, the solar heat collector tube 10 of Embodiment 2, thesolar heat collector tube 20 of Embodiment 3 and the solar heatcollector tube 30 of Embodiment 4, the solar heat collector tube 40 ofthe present embodiment having such features allows prevention ofimpairment in the function of the layer overlying the tube 2.

Embodiment 6

FIG. 12 is a partial cross-sectional diagram of solar heat collectortube of the present embodiment.

In FIG. 12, a solar heat collector tube 50 of the present embodimentdiffers from the solar heat collector tube 40 of Embodiment 5 in that areaction preventing layer 14 is provided between the metal protectivelayer 11 and the oxygen barrier layer 12. Other features are identicalto those of the solar heat collector tube 40 of Embodiment 5, andaccordingly will not be explained. The features of the presentembodiment can also apply to the solar heat collector tube 30 ofEmbodiment 4.

In a case where the metal protective layer 11 is formed of a metal suchas Nb, Mo, W, Cu, Ni, Fe, Cr, Ta or the like, the metal protective layer11 may react with the oxygen barrier layer 12. The reaction preventinglayer 14 is provided in order to prevent such reactions between themetal protective layer 11 and the oxygen barrier layer 12. Impairment ofthe functions of the metal protective layer 11 and of the oxygen barrierlayer 12 becomes accordingly less likely by providing the reactionpreventing layer 14 between the metal protective layer 11 and the oxygenbarrier layer 12.

Even if the oxygen barrier layer 12 is not formed, the reactionpreventing layer 14 allows prevention of reactions between the metalprotective layer 11 and the sunlight-heat conversion layer 4. Thereaction preventing layer 14 also has the function of preventing passageof oxygen, and accordingly can substitute for the oxygen barrier layer12 in this case.

The reaction preventing layer 14 is not particularly limited, so long asreactions between the metal protective layer 11 and the oxygen barrierlayer 12 or the sunlight-heat conversion layer 4 are less likely, and alayer known in the relevant technical field can be used as the reactionpreventing layer 14. Examples of materials utilized in the reactionpreventing layer 14 include, for instance, silicides such as niobiumsilicide (NbSi₂) and tantalum silicide (TaSi₂). Among the foregoing, thematerial of the reaction preventing layer 14 is preferably a silicide ofa metal that is used in the metal protective layer 11.

The thickness of the reaction preventing layer 14 is not particularlylimited so long as reactions between the metal protective layer 11 andthe oxygen barrier layer 12 or the sunlight-heat conversion layer 4 canbe prevented, but is generally 1 nm to 200 nm, preferably 3 nm to 100 nmand more preferably 5 nm to 80 nm.

The method for forming the reaction preventing layer 14 is notparticularly limited, and a method known in the relevant technical fieldcan be resorted to. For instance, the reaction preventing layer 14 canbe formed by chemical vapor deposition or physical vapor deposition(sputtering, vacuum deposition or ion plating).

In addition to the effect of the solar heat collector tube 30 ofEmbodiment 4 and the solar heat collector tube 40 of Embodiment 5, thesolar heat collector tube 50 of the present embodiment having suchfeatures allows prevention of impairment of the function of the metalprotective layer 11 and the oxygen barrier layer 12 or the sunlight-heatconversion layer 4.

In a case where the metal protective layer 11 is formed of a compound ofthe metal (at least one metal selected from the group consisting of Mo,W, Ta, Nb and Al) dispersed in the Ag layer 7 and silicon or nitrogen(for instance, TaSi₂, MoSi₂, Mo₅Si₃, WSi₂, TaN, NbSi₂ or NbN), the metalprotective layer 11 and the oxygen barrier layer 12 do not reactreadily, and accordingly it is not necessary to provide the reactionpreventing layer 14 between the metal protective layer 11 and the oxygenbarrier layer 12. Therefore, in a case where the metal protective layer11 is formed from a compound of the metal dispersed in the Ag layer 7and silicon or nitrogen, an effect identical to that of the solar heatcollector tube 50 of the present embodiment can be achieved, even if thereaction preventing layer 14 is not provided, and also productivity canbe increased since it is not necessary to provide the reactionpreventing layer 14.

Embodiment 7

FIG. 13 is a partial cross-sectional diagram of a solar heat collectortube of the present embodiment.

In FIG. 13, a solar heat collector tube 60 of the present embodimentdiffers from the solar heat collector tube 50 of Embodiment 6 in that areaction preventing layer 14 is provided between the diffusionpreventing layer 13 and the metal protective layer 11. Other featuresare identical to those of the solar heat collector tube 50 of Embodiment6, and accordingly will not be explained. The features of the presentembodiment can apply also to the solar heat collector tube 40 ofEmbodiment 5.

In a case where the metal protective layer 11 is formed of a metal suchas Nb, Mo, W, Cu, Ni, Fe, Cr, Ta or the like, the metal protective layer11 may react with diffusion preventing layer 13. The reaction preventinglayer 14 that is provided between the diffusion preventing layer 13 andthe metal protective layer 11 is provided in order to prevent suchreactions between the diffusion preventing layer 13 and the metalprotective layer 11. Accordingly, impairment of the functions of thediffusion preventing layer 13 and the metal protective layer 11 becomesless likely by providing the reaction preventing layer 14 between thediffusion preventing layer 13 and the metal protective layer 11.

A reaction preventing layer identical to the reaction preventing layer14 that is provided between the metal protective layer 11 and the oxygenbarrier layer 12 can be used as the reaction preventing layer 14 that isprovided between the diffusion preventing layer 13 and the metalprotective layer 11. Among the foregoing, the material of the reactionpreventing layer 14 is preferably a silicide of a metal that is used inthe metal protective layer 11.

The thickness of the reaction preventing layer 14 that is providedbetween the diffusion preventing layer 13 and the metal protective layer11 is not particularly limited, so long as reactions between thediffusion preventing layer 13 and the metal protective layer 11 can beprevented, but is generally 1 nm to 150 nm, preferably 5 nm to 100 nmand more preferably 10 nm to 80 nm.

Here, the stack in FIG. 14 is produced through sequential layering, on aquartz substrate, of the reaction preventing layer 14 (20 nm TaSi₂layer), the metal protective layer 11 (20 nm Ta layer), the Ag layer 7(230 nm) having dispersed therein 1.25 at % of silicon, silicon nitrideor a mixture thereof 6, the metal protective layer 11 (10 nm Ta layer),the reaction preventing layer 14 (10 nm TaSi₂ layer) and the oxygenbarrier layer 12 (50 nm Si₃N₄ layer). FIG. 15 illustrates the results oflight transmittance of the stack before and after heating of the stackat 700° C. for 1 hour, 11 hours and 51 hours. As FIG. 15 reveals, thelight transmittance of the stack exhibits virtually no change before orafter heating. By adopting such a multilayer structure, therefore, thefunctions of the various layers are not impaired and the efficiency ofconversion of sunlight to heat does not drop.

In addition to the effect of the solar heat collector tube 40 ofEmbodiment 5 and of the solar heat collector tube 50 of Embodiment 6,the solar heat collector tube 60 of the present embodiment having suchfeatures allows prevention of impairment of the functions of the metalprotective layer 11 and the diffusion preventing layer 13.

In a case where the metal protective layer 11 is formed of a compound ofthe metal (at least one metal selected from the group consisting of Mo,W, Ta, Nb and Al) dispersed in the Ag layer 7 and silicon or nitrogen(for instance, TaSi₂, MoSi₂, Mo₅Si₃, WSi₂, TaN, NbSi₂ or NbN), thediffusion preventing layer 13 and the metal protective layer 11 do notreact readily, and accordingly it is not necessary to provide thereaction preventing layer 14 between the diffusion preventing layer 13and the metal protective layer 11. Therefore, in a case where the metalprotective layer 11 is formed from a compound of the metal dispersed inthe Ag layer 7 and silicon or nitrogen, an effect identical to that ofthe solar heat collector tube 60 of the present embodiment can beachieved, even if the reaction preventing layer 14 is not provided, andalso productivity can be increased since it is not necessary to providethe reaction preventing layer 14.

The present international application claims priority based on JapanesePatent Application No. 2016-015519, filed on Jan. 29, 2016, the entirecontents whereof being incorporated herein by reference.

REFERENCE SIGNS LIST

1, 10, 20, 30, 40 Solar heat collector tube

2 Tube

3 Infrared reflective layer

4 Sunlight-heat conversion layer

5 Anti-reflection layer

6 Silicon, silicon nitride or a mixture thereof

7 Ag layer

11 Metal protective layer

12 Oxygen barrier layer

13 Diffusion preventing layer

14 Reaction preventing layer

The invention claimed is:
 1. A solar heat collector tube in which atleast an infrared reflective layer, a sunlight-heat conversion layer andan anti-reflection layer are provided on the outer surface of a tube,through the interior of which a heat medium can flow, wherein theinfrared reflective layer is an Ag layer in which silicon, siliconnitride, or a mixture thereof is dispersed, wherein a metal protectivelayer is provided between the infrared reflective layer and thesunlight-heat conversion layer, wherein the metal protective layer isformed by at least one metal selected from the group consisting of Mo,W, Ta, and Nb, or the metal protective is layer is formed by a compoundof silicon or nitrogen with at least one metal selected from the groupconsisting of Mo, W, Ta, and Nb, wherein a content of the silicon, thesilicon nitride, or the mixture thereof in the infrared reflective layeris from 0.1 to lower than 10 at %, and wherein at least one metalselected from the group consisting of Mo, W, Ta, Nb, and Al is furtherdispersed in the Ag layer.
 2. The solar heat collector tube of claim 1,wherein another metal protective layer is provided between the tube andthe infrared reflective layer.
 3. The solar heat collector tube of claim1, wherein the metal protective layer is formed of a material containingthe metal that is dispersed in the Ag layer.
 4. The solar heat collectortube of claim 1, wherein the metal protective layer is formed of acompound of silicon or nitrogen, and the metal dispersed in the Aglayer.
 5. The solar heat collector tube of claim 1, wherein an oxygenbarrier layer is provided between the metal protective layer and thesunlight-heat conversion layer.
 6. The solar heat collector tube ofclaim 1, wherein a diffusion preventing layer is provided between thetube and the infrared reflective layer.
 7. The solar heat collector tubeof claim 5, wherein a reaction preventing layer is provided between themetal protective layer and the oxygen barrier layer or the sunlight-heatconversion layer.
 8. The solar heat collector tube of claim 6, wherein areaction preventing layer is provided between the diffusion preventinglayer and the metal protective layer.
 9. A method for producing a solarheat collector tube in which at least an infrared reflective layer, ametal protective layer, a sunlight-heat conversion layer and ananti-reflection layer are provided on the outer surface of a tube,through the interior of which a heat medium can flow, the methodcomprising: forming the infrared reflective layer that is an Ag layer inwhich silicon, silicon nitride or a mixture thereof is dispersed, bysputtering in the presence of a gas including nitrogen gas, with Ag andsilicon being used as targets, wherein the metal protective layer isformed by at least one metal selected from the group consisting of Mo,W, Ta, and Nb, or the metal protective layer is formed of a compound ofsilicon or nitrogen with at least one metal selected from the groupconsisting of Mo, W, Ta, and Nb, wherein a content of the silicon, thesilicon nitride, or the mixture thereof in the infrared reflective layeris from 0.1 to lower than 10 at %, and wherein at least one metalselected from the group consisting of Mo, W, Ta, Nb, and Al is furtherdispersed in the Ag layer.
 10. The solar heat collector tube of claim 5,wherein a reaction preventing layer is provided between the metalprotective layer and the oxygen barrier layer.
 11. The heat solar heatcollector tube of claim 1, wherein a content of the silicon, the siliconnitride or the mixture thereof in the infrared reflective layer is from0.1 to 5 at %.
 12. The method of claim 9, wherein a content of thesilicon, the silicon nitride or the mixture thereof in the infraredreflective layer is from 0.1 to 5 at %.